Articles | Volume 12, issue 1
Research article
05 Jan 2015
Research article |  | 05 Jan 2015

Spatiotemporal variations of nitrogen isotopic records in the Arabian Sea

S.-J. Kao, B.-Y. Wang, L.-W. Zheng, K. Selvaraj, S.-C. Hsu, X. H. Sean Wan, M. Xu, and C.-T. Arthur Chen

Abstract. Available reports of dissolved oxygen, δ15N of nitrate (δ 15NNO3) and δ15N of total nitrogen (δ15Nbulk) for trap material and surface/downcore sediments from the Arabian Sea (AS) were synthesized to explore the AS' past nitrogen dynamics. Based on 25 μmol kg−1 dissolved oxygen isopleth at a depth of 150 m, we classified all reported data into northern and southern groups. By using δ15Nbulk of the sediments, we obtained geographically distinctive bottom-depth effects for the northern and southern AS at different climate stages. After eliminating the bias caused by bottom depth, the modern-day sedimentary δ15Nbulk values largely reflect the δ15NNO3 supply from the bottom of the euphotic zone. Additionally to the data set, nitrogen and carbon contents vs. their isotopic compositions of a sediment core (SK177/11) collected from the most southeastern part of the AS were measured for comparison. We found a one-step increase in δ15Nbulk starting at the deglaciation with a corresponding decrease in δ13CTOC similar to reports elsewhere revealing a global coherence. By synthesizing and reanalyzing all reported down core δ15Nbulk, we derived bottom-depth correction factors at different climate stages, respectively, for the northern and southern AS. The diffusive sedimentary δ15Nbulk values in compiled cores became confined after bias correction revealing a more consistent pattern except recent 6 ka. Such high similarity to the global temporal pattern indicates that the nitrogen cycle in the entire AS had responded to open-ocean changes until 6 ka BP. Since 6 ka BP, further enhanced denitrification (i.e., increase in δ15Nbulk) in the northern AS had occurred and was likely driven by monsoon, while, in the southern AS, we observed a synchronous reduction in δ15Nbulk, implying that nitrogen fixation was promoted correspondingly as the intensification of local denitrification at the northern AS basin.

Short summary
This paper presents a new sedimentary nitrogen isotope record (d15N) of a sediment core from the southeastern Arabian Sea (AS). By compiling the published nitrogen isotope data in the AS, we obtain geographically distinctive bottom-depth effects for the northern and southern AS since 35ka. After eliminating the bottom-depth bias, we observe opposite d15N trends in the Holocene between these two areas, reflecting a special coupling of denitrification to the north and N2-fixation to the south.
Final-revised paper